Core-binding factors (CBFs) are heterodimeric transcription factors that play critical roles both in mammalian development and in human disease. CBFs consist of DNA-binding CBFalpha subunits encoded by the Runx1, Runx2, or Runx3 genes, and a non-DNA-binding CBFBeta subunit encoded by the Cbfb gene. Runx] and Cbtb are required for hematopoiesis, Runx-2 for bone formation, and Runx3 for stomach and sensory neuron development. This proposal focuses on the CBFBeta subunit, which is the common non-DNA binding partner for all of the CBFalpha proteins. The CBFBeta subunit increases the affinity of CBFalpha subunits for DNA, and is essential for both Runx1 and Runx2 function in vivo. CBFBeta becomes fused to sequences from the smooth muscle myosin heavy chain (SMMHC) as a result of the inv(16)(p13;q22) in acute myeloid leukemia. The CBFBeta-SMMHC fusion protein functions a dominant inhibitor of CBFBeta function, and by this mechanism contributes to leukemia. A novel, high affinity binding site for Runx1 in CBFBeta-SMMHC has been identified that may be responsible for its potent dominant negative activity. This proposal focuses both on the function of the normal CBFbeta protein, and how its fusion to SMMHC sequences disrupts that function. Specifically, we plan to: 1. Characterize the high-affinity Runx1 binding site in the CBFBeta-SMMHC protein, and determine if that site is required for the dominant negative activity of CBFBeta-SMMHC. 2. Determine the mechanism by which CBFBeta allosterically regulates DNA binding by Runx1, and if CBFBeta interacts with other proteins to regulate transcription. 3. Determine if CBFBeta is required for Runx3 function in vivo.